A circuit interrupter is an electrical component that can break an electrical circuit, interrupting the current. A basic example of a circuit interrupter is a switch, which generally consists of two electrical contacts in one of two states; either closed meaning the contacts are physically touching and electrical current pass from one contact to the other, or open, meaning the contacts are separated relative to each other thereby preventing the flow of electrical current there between. A switch may be directly manipulated by a person as a control signal to a system, such as a computer keyboard button, or to control power flow in a circuit, such as a light switch.
A circuit breaker can be used as a replacement for a fuse. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Fuses perform much the same function as circuit breakers, however, circuit breakers are typically safer to use and may be reset after tripping. If a fuse blows, a person will often have to closely examine the fuses to determine which fuse in particular was burned or spent. The fuse will then have to be removed from the fuse box, discarded and a new fuse will have to be installed.
Circuit breakers are much easier to operate than fuses. When a circuit breaker trips, one can easily look at the electrical panel and see which breaker handle has moved to the tripped position. The circuit breaker can then be “reset” by turning the handle to the “off” position, and then moved to the “on” position. In general, a circuit breaker has two contacts located inside of a housing. The first contact is typically stationary, and may be connected to either the “line” side connection (connection to the power supply) or the “load” side connection (connection to the device to be powered). The second contact is movable with respect to the first contact, such that when the circuit breaker is in the “off”, or tripped position, a physical gap exists between the first and second contacts.
To trip the circuit breaker so as to “open” the circuit, a solenoid with an overcurrent sensor may be used. When the overcurrent sensor senses a specific current level, or a percentage above the rated current of the circuit breaker, the solenoid may be actuated to mechanically move an arm thereby tripping the circuit breaker to open the circuit.
To prevent the circuit breaker from accidentally tripping, the tripping mechanism can be set to a relatively high level, so that a small current spike would not result in the tripping of the circuit breaker. However, this configuration is disadvantageous in that in the event of a relatively small over current for an extended period of time, the circuit breaker would not trip. This is undesirable as it could lead to damage of the electrical distribution system itself and to equipment connected to the distribution system.
Instead of setting the tripping mechanism at a high current level, many circuit breakers have a delayed tripping mechanism so that the circuit breaker only trips after the detection of an over current condition for a specific period of time. This prevents the circuit breaker from immediately tripping, thus preventing many situations where the circuit breaker would be accidentally tripped upon the detection of a relatively low current spike (e.g., startup of a motor), but would also protect the equipment from a low over current condition that lasts for an extended period of time (e.g., the motor windings become damaged and are beginning to short). However, the introduction of the delay prevents the circuit breaker from immediately tripping when a dangerous high current spike or short, occurs, which can severely damage both the electrical distribution system itself and the connected equipment. For example, in situation where a ground fault occurs, a person may accidentally come into contact with electrical current, which any delay in the tripping of the circuit breaker corresponds to an increase in the amount of time the person is in contact with the live electrical circuit leading to severe injury, or even death. Likewise, during a short circuit condition, if the circuit breaker is delayed before tripping, the equipment connected to the electrical circuit may be severely damaged. However, current limiting circuit breakers with a built-in time delay generally do not trip until after the angle θ of the sine wave reaches zero. Accordingly, in one situation where a large current spike occurs just after the zero crossing, the delay in the breaker tripping can approach the time it takes for the sine wave to cycle 180°, which is unacceptable. The removal of even a few milliseconds from this delay can be crucial to avoid severe personal injury or permanently damaging connected equipment.
However, the concept of inverse time tripping in and of itself is well known. Inverse time tripping is a characteristic of circuit breakers where the breaker trips more slowly with lower overcurrent, and more quickly I with higher overcurrent. As an example, Article 100 of the National Electric Code (NEC) states: “Inverse Time (as applied to circuit breakers). A qualifying term indicating that there is purposely introduced a delay in the tripping action of the circuit breaker, which delay decreases as the magnitude of the current increases.”
The inverse time is typically achieved by attaching some mechanical accessories in the circuit breaker. In one example, it is achieved in an induction disc relay by positioning a permanent magnet such that, when the disc rotates, it cuts the flux of a permanent magnet. A current is then induced in the disc, which slows down the movement of the disc. A circuit breaker can be made an inverse time breaker, by providing a piston and an oil dash-pot. A piston that is attached to the moving iron plunger, is immersed in the oil in a dash-pot. When the solenoid relay is actuated, the piston moves upwards along with iron plunger wherein the viscosity of the oil slows the upward movement of plunger. The speed of this upward movement against gravity depends upon how strongly the solenoid attracts the iron plunger. The attraction force is determined based on the magnitude of actuating current. This results in a time of operation of the breaker that is inversely proportional to the actuating current.
However, this configuration has the disadvantage of being bulky, requiring many complex components including an oil dash-pot, which may be subject to leaking. The complex and numerous parts and required maintenance result in a relatively expensive device that is not appropriate for many installations.
Also known are thermal magnetic circuit breakers that utilize techniques where electromagnet components respond quickly to large surges in current and a bi-metallic strip responds more slowly to lower over-currents situations. The thermal portion of the circuit breaker provides an “inverse time” response feature, which trips the circuit breaker more quickly for a larger overcurrent situation, but allows a smaller overcurrent to persist for a longer time before opening.
A major disadvantage of the bi-metallic strip is that the rate of heating and cooling of the bi-metal is affected by ambient temperature, the performance of the breaker differs for different ambient temperatures. While this major drawback can be addressed somewhat by use of a resistance temperature detector (RTD), this again leads to a complex device having numerous parts and requires a controller to control the RTD leading to a relatively expensive device and is simply not appropriate for many installations.
Also known in the prior art is U.S. Pat. No. 8,749,329 entitled “Magnetic Circuit Interrupter With Current Limiting Capability” to Michael Fasano. This reference discloses two separate trip mechanisms each comprising a solenoid that are designed to actuate at different current levels. A major benefit that this system provides for is that it allows for quick tripping for a relatively large current spike but would not trip for a relatively small current spike. Additionally, the functioning of the system is not dependent on ambient temperature.
However, a limitation of the above-listed references relates to the speed at which the system can react to overcurrent conditions. For example, it is understood that very small amounts of time (milliseconds) can make the difference in whether a device connected to the system in an overcurrent condition is damaged and/or destroyed or whether the device is safely disconnected without damage.